1. Field of Invention
The present invention relates to a method for updating ephemeris data of global positioning system (GPS). More particularly, the present invention relates to a method for updating GPS ephemeris data, which may be automatically downloaded from a GPS ephemeris data server.
2. Description of Related Art
Global Positioning System (hereafter denoted as GPS), a precise satellite navigation positioning technology, has been developed by the United States of America. In the global satellite positioning system, there are 26 satellites distributed on six orbits which are inclined at fifty-five degrees with respect to the equatorial plane, so that the users of the global satellite positioning system, no matter when or where on the earth, as long as no shielding of terrains or buildings, can receive the signals from 4 to 8 satellites so as to be positioned by the satellites without the influence of weather. Taking advantage of the advanced technology, anyone can easily get the correct position, velocity and time from his/her GPS navigation device.
Funded by the U.S. Department of Defense and participated by the US Department of Transportation, GPS development plan has been transferred for civil uses, such as the relative operations of navigation positioning, precise measuring, attitude controlling and standard timing. As a navigation system combining the technologies of satellite and radio, GPS can provide user's precise position, velocity and time. The application of GPS has been extensively existed in the current market and, along with the progress of the satellite technology, a great deal of new GPS technologies are being rapidly developed as well as business opportunities. In the early stages, GPS was limited to military sector for military uses, such as precise positioning of warplanes, warships, vehicles, personnel and attack targets. Nowadays, GPS has been opened to civil positioning use. The business of this high tech combining the technologies of satellite and radio are thriving in the extensively civil market such as providing exact information about velocity, time, direction and distance as well as precise positioning. GPS is indeed full of unlimited business opportunities.
However, in the present GPS positioning, for receiving satellite signals and downloading GPS ephemeris data from GPS satellites through the GPS antenna, the navigation device has to be located at an open area such as sky and no shielding around. Only when GPS ephemeris data has been downloaded, the needed ephemeris data is enough to perform the first positioning process. The process above is called ‘cold start’. Such a process takes from 45 seconds to 2 minutes. If the user is in an area where GPS signals are not received well, a longer time will be taken.
Still and all, those downloaded GPS ephemeris data is in timeliness. The locations of GPS satellites in the space are not fixed as the GPS satellite revolves around the earth two times a day. For example, with the SiRF GPS module, all the GPS ephemeris data is effective in four hours. The original GPS ephemeris data will be no value for positioning after four hours, if no update action is done during this time. Therefore, it is necessary to perform the cold start process once more. In this way, it takes a lot of time to update GPS ephemeris data periodically.
In view of this, Global System for Mobile Communications(GSM) integrated with the General Packet Radio Service(GPRS), applied in the cell phones have been developed for positioning in recent years, which generally be called as AGPS(assist). The biggest difference between the traditional GPS and AGPS is that GPS only uses satellites to do positioning and takes a long time for the first and latter positioning while AGPS roughly searches the user's position through the GSM base station first, then do precise positioning through GPS satellites. Furthermore, AGPS not only saves time from the traditional one minute to 10 seconds for the first positioning and from 40 seconds to 0.1 second for each latter positioning, but also overcomes the weakness that the traditional GPS can not do positioning in the house due to the bad satellite signals therein.
AGPS can overcome the weakness that the traditional GPS positioning takes long time to update GPS ephemeris data, however, the cell phone with AGPS functions is indispensable first for the user. Then, the telecommunication operator must provide further service. Besides, the telecommunication operator will charge for the transmission of AGPS packages, so that it is not cost-efficient.